Packet-based networks, such as those based on Internet Protocol (IP) or Asynchronous Transfer Mode (ATM) technologies, are frequently being used to deliver real-time voice and video services. Virtually all telecommunication services—including those using wire, optical fiber or wireless transport—are expected to be packet based within the next few years.
All packet-based networks are subject to packet loss. For example, packet loss occurs when either a router or other piece of hardware becomes temporarily unable to meet its bandwidth demand, or when interference occurs in a wireless link. For real-time applications such as telephony, packet loss also occurs when a packet fails to reach its destination within some specified period of time. Various strategies are in place to deal with packet loss. Foremost, careful network planning can be used to avoid congestion and reduce overall loss rates. Although critical, this alone is not enough, and often means costly over-provisioning to handle peak demand times. As such, Quality-of-Service (QoS) metrics can be used to prioritize packets, and relegate loss to less critical or non-real-time streams of information; the Transmission Control Protocol is used with IP (TCP/IP) to request that lost packets be resent when time allows; and Packet loss concealment (PLC) or Error Concealment (EC) strategies are used in voice applications to minimize the perceptual effect of lost packets on the end-user (e.g., see International Telecommunications Union (ITU)-T G.711 Appendix I). In addition, voice and video encoding algorithms that are inherently more resilient to loss can be used.
Ultimately, the success or failure of any scheme used to conceal error will depend on an end-users perception of transmission quality. Due to both cognitive and algorithmic factors, the perception of quality is dependent upon the patterns of loss observed on the network. It has long been observed that loss on packet-based networks is bursty. That is, the loss tends to occur such that sequential strings of packets are lost. Accordingly, many models of packet loss are based upon a 2-state Markov model, which can account for the observation that a packet is more likely to be lost if the immediately preceding packet was lost.
As such, the burstiness of loss is critical in assessing the performance of methods to minimize loss effects, whether they are PLC schemes or advanced routing algorithms. Unfortunately, there is no clear metric of how bursty a network is.